1. Field of the Invention
The present invention relates to a field effect transistor such as a metal semiconductor field effect transistor (MESFET) having comb-shaped lead-out electrodes.
2. Description of the Related Art
Generally, in a mobile telephone set, MESFETs using Schottky junction gates have been used as high speed switches for switching antennas which receive and transmit about 1 to 2 GHz signals.
A prior art MESFET uses comb-shaped electrodes in order to decrease the ON resistance. That is, the prior art MESFET is constructed by a semiconductor substrate which is divided into an active area and an inactive area, a comb-shaped gate electrode having a trunk portion formed on the inactive area and gate fingers formed on the active area, source ohmic electrodes and drain ohmic electrodes formed on the active area and alternating with the gate fingers of the comb-shaped gate electrodes, a comb-shaped source lead-out electrode having a trunk portion formed on the inactive area and fingers each connected to one of the source ohmic electrodes and formed on the active area, and a comb-shaped drain lead-out electrode having a trunk portion formed on the inactive area and fingers each connected to one of the drain ohmic electrodes and formed on the active area. In this case, the edges of the fingers of the comb-shaped source lead-out electrode are in proximity to the edges of the source ohmic electrodes, and the edges of the fingers of the comb-shaped drain lead-out electrode are in proximity to the edges of the drain ohmic electrodes. In other words, the comb-shaped source lead-out electrode and the comb-shaped drain lead-out electrode are completely interdigitated. This will be explained later in detail.
In the above-described prior art MESFET, however, since the comb-shaped source lead-out electrode and the comb-shaped drain lead-out electrode are completely interdigitated so that the opposing amount therebetween is very large, the parasitic capacitance between the lead-out electrodes is remarkably increased, which would decrease the operation speed.
It is an object of the present invention to provide a field effect transistor including comb-shaped electrodes capable of increasing the operation speed.
According to the present invention, in a field effect transistor including a semiconductor substrate which is divided into an active area and an inactive area, a comb-shaped gate electrode having a trunk portion formed on the inactive area and gate fingers formed on the active area, source ohmic electrodes and drain ohmic electrodes formed on the active area and alternating with the gate fingers of the comb-shaped gate electrodes, a comb-shaped source lead-out electrode having a trunk portion formed on the inactive area and fingers each connected to one of the source ohmic electrodes and formed on the active area, and a comb-shaped drain lead-out electrode having a trunk portion formed on the inactive area and fingers each connected to one of the drain ohmic electrodes and formed on the active area, edges of the fingers of the comb-shaped source lead-out electrode recede from edges of respective ones of the source ohmic electrodes, or edges of the fingers of the comb-shaped drain lead-out electrode recede from edges of respective ones of the drain ohmic electrodes.
Thus, the opposing amount between the lead-out electrodes is decreased to reduce the parasitic capacitance therebetween.